1. Field of the Invention
The present invention relates to a magnetooptical information recording medium and a method for regenerating information from said medium.
2. Related Background Art
There are already known magnetooptical information recording media utilizing a polycrystalline thin layer for example of MnBi or MnCuBi, an amorphous thin layer for example of GdCo, GdFe, TbFe, DyFe, GdTbFe or TbDyFe, or a crystalline thin layer for example of GIG. Among these materials, a thin layer of rare earth transition metal amorphous alloys is recently considered adequate for producing a magentooptical information recording medium, in consideration of ease of producing a thin layer of a large area at a temperature close to the normal temperature.
In general, the magnetooptical information recording medium is required to show a high recording sensitivity, a large magnetooptical effect and a high coercive force. However it has been difficult to meet all these requirements with any of the above-mentioned thin layers alone. For example GdCo or GdFe, allowing compensation point recording, shows a large magnetooptical effect at information reading and provides a high S/N ratio due to a relatively high Curie temperature, but shows a low coercive force so that the recorded domains are unstable. On the other hand, TbFe or DyFe allowing Curie point recording is not associated with the above-mentioned drawbacks because of a relatively high coercive force, but provides a low S/N ratio at information reading due to the low Curie temperature. In order to avoid these drawbacks, there is proposed, for example in the Japanese Patent Laid-open No. 153546/1981 etc., a magnetooptical information recording medium of a two-layered structure in which a reading layer with a large magnetooptical effect and a low coercive force is magnetostatically combined with a recording layer with a low Curie temperature and a high coercive force. In this medium, the domains recorded in the recording layer are transferred, by magnetostatic coupling, to the reading layer, from which the information is read by a magnetooptical effect.
However, in such two-layered magnetooptical information recording medium, the magnetostatic coupling achieved by an interaction of the domains in said layers through a floating magnetic field and is locally limited to the portion where the domains exist, so that the stability of direction of magnetization of the recording layer is questionable where the recording layer does not have domains. Also the magnitude of the floating magnetic field resulting from the domains of the recording layer is often insufficient for transferring domains in the reading layer.